Sprouts Farmers Market, Inc. (NASDAQ: SFM) has had serious competition since Whole Foods Market was acquired by Amazon.com (NASDAQ: AMZN), and the e-commerce giant slashed Whole Foods' prices near the end of August. At that same time, Sprouts' stock price dropped in 10 days from $21.44 per share on Aug. 18 to $19.72 on Aug. 28.

According to a Thasos Group analysis of location data from millions of mobile phone users, Whole Foods' customer traffic spiked 31 percent from a year earlier on Aug. 28.

Where did those customers come from? According to the report, Trader Joe's and Sprouts with 10 percent of Trader Joe’s regular customers visiting Whole Foods between Aug. 28 and Sept. 3, followed by Sprouts, with 8 percent.

Despite that increased competition, Sprouts is still pushing for growth. The Phoenix-based grocer announced to open 30 new stores in 2018, including nine during the first quarter.

Two of those new locations are in the Phoenix metro, with sites slated for Mesa and Tempe.